U.S. Pat. No. 4,971,932 (Alpha et al.) is generally related to the construction of magnetic memory storage devices, the essential components of which are a head pad and a rigid information disc. As is described in that patent, the information disc consists of two basic elements: (a) a rigid substrate, and (b) a coating of magnetic media on the surface of the substrate facing the head pad. The disclosure of that patent is directed specifically to the composition and structure of the rigid substrate.
Thus, the inventive concept encompassed within the patent comprised the fabrication of the substrate from two different groups of glass-ceramic materials:
(1) glass-ceramics wherein crystals exhibiting a sheet silicate structure constitute the predominant crystal phase; and
(2) glass-ceramics wherein crystals exhibiting a chain silicate structure constitute the predominant crystal phase.
The patent cited seven characteristics of those glass-ceramics which rendered them especially suitable for high performance rigid disc substrates:
(a) high body strengths and fracture toughnesses, as evidenced by moduli of rupture between about 15,000-40,000 psi (.about.105-280 MPa) and K.sub.IC values between about 3-5 MPa.sqroot.m, respectively;
(b) an elastic modulus at least equal to, and preferably higher than that of aluminum metal (nickel phosphorus plated aluminum substrates have been used extensively in information discs) such that the material is dimensionally stable at all thicknesses and rotational velocities;
(c) a high surface hardness to impart excellent scratch resistance;
(d) the capability of forming precision surfaces (flat and smooth);
(e) a linear coefficient of thermal expansion between about 75-125.times.10.sup.-7 /.degree.C. over the temperature range of 25.degree.-300.degree. C.;
(f) excellent resistance to attack in moist environments; and
(g) an inherently textured surface of a desired roughness (R.sub.a =0.5-5 nm) for enhanced magnetics and lower flying height.
As was disclosed in U.S. Pat. No. 4,971,932, two composition areas within the general system yielding glass-ceramics containing synthetic fluormica crystals with sheet silicate structures have been investigated for their utility as substrates in rigid information discs: (a) compositions wherein fluorophlogopite solid solution comprises the predominant crystal phase, and (b) compositions wherein tetrasilicic fluormica constitutes the predominant crystal phase.
U.S. Pat. No. 3,689,293 (Beall) discloses the preparation of glass-ceramic articles wherein fluorophlogopite solid solution comprises the predominant crystal phase, those articles being suitable for the fabrication of substrates for rigid information discs. Those glass-ceramics consist essentially, expressed in terms of weight percent on the oxide basis, of:
______________________________________ SiO.sub.2 25-60 Cs.sub.2 O 0-20 B.sub.2 O.sub.3 5-15 Na.sub.2 O + K.sub.2 O + Rb.sub.2 O + Cs.sub.2 O 2-20 Al.sub.2 O.sub.3 5-25 MgO 4-25 B.sub.2 O.sub.3 + Al.sub.2 O.sub.3 15-35 Li.sub.2 O 0-7 Na.sub.2 O 0-15 MgO + Li.sub.2 O 6-25 K.sub.2 O 0-15 F 4-20 Rb.sub.2 O 0-15 ______________________________________
One glass-ceramic having a composition coming within the above ranges which has been found useful for forming substrates for rigid information discs is a product marketed commercially by Corning Incorporated, Corning, N.Y. as Corning 9658 under the trademark MACOR. That glass-ceramic has the following approximate composition (O.dbd.F is the oxygen.perspectiveto.fluoride correction factor):
______________________________________ SiO.sub.2 47.0 K.sub.2 O 9.5 B.sub.2 O.sub.3 8.5 F 6.3 Al.sub.2 O.sub.3 16.7 O.dbd.F 2.5 MgO 14.5 ______________________________________
U.S. Pat. No. 3,732,087 (Grossman) discloses the preparation of glass-ceramic articles wherein tetrasilicic fluormica comprises the predominant crystal phase, those articles also being suitable for the formation of substrates for rigid information discs. Those glass-ceramics consist essentially, expressed in terms of weight percent on the oxide basis, of 45-70% SiO.sub.2, 8-20% MgO, 8-15% MgF.sub.2, 5-25% R.sub.2 O, wherein R.sub.2 O consists of 0-20% K.sub.2 O, 0-20% Rb.sub.2 O, and 0-20% Cs.sub.2 O, 0-20% RO, wherein RO consists of 0-20% SrO, 0-20% BaO, and 0-20% CdO, and 5-35% R.sub.2 O+RO.
The preparation of substrates for rigid information discs from glass-ceramics containing a chain silicate as the predominant crystal phase has been preferred, however, and of such glass-ceramics three specifically-defined composition areas have been found to be particularly effective:
(1) glass-ceramics wherein canasite constitutes the predominant crystal phase;
(2) glass-ceramics wherein potassium fluorrichterite constitutes the predominant crystal phase; and
(3) glass-ceramics wherein potassium fluorrichterite constitutes the predominant crystal phase, but wherein a substantial amount of cristobalite is also present.
Thus, highly crystalline glass-ceramics consisting of randomly-oriented, tightly interlocked, high aspect ratio crystals of canasite or fluorrichterite are produced by means of the controlled nucleation and crystallization of precursor glasses of predetermined compositions. It is the crystalline microstructure of these glass-ceramics which imparts the very high strength and toughness thereto, as well as a surface texture of desirable roughness.
U.S. Pat. No. 4,386,162 (Beall) describes the formation of glass-ceramic bodies containing canasite and/or agrellite and/or fedorite as the predominant crystal phase. Glass-ceramics satisfying the seven criteria set forth above for substrates of rigid information discs have been prepared via heat treatment of parent glass bodies having the following compositions, expressed in terms of weight percent on the oxide basis, yielding articles wherein canasite comprises the predominant crystal phase:
______________________________________ SiO.sub.2 50-70 F 4-9 CaO 15-25 MgO 0-2 Na.sub.2 O 6-10 ZnO 0-2 K.sub.2 O 6-12 SnO.sub.2 0-2 Al.sub.2 O.sub.3 1-4 Sb.sub.2 O.sub.3 0-1 ______________________________________
U.S. Pat. No. 4,467,039 (Beall et al.) discloses glass-ceramics wherein potassium fluorrichterite constitutes the predominant crystal phase. Glass-ceramics encompassed within that disclosure satisfying the above criteria have been produced through heat treating glass articles having compositions within the intervals below, expressed in terms of weight percent on the oxide basis, to form articles wherein potassium fluorrichterite comprises essentially the sole crystal phase:
______________________________________ SiO.sub.2 58-70 K.sub.2 O 3.8-7 Al.sub.2 O.sub.3 0.5-3 Li.sub.2 O 0.5-2 MgO 13-17.5 BaO 0-2.5 CaO 3-7 P.sub.2 O.sub.5 0-2.5 Na.sub.2 O 1.5-4 F 3-5.5 ______________________________________
U.S. Pat. No. 4,608,348 (Beall et al.) discusses glass-ceramics containing potassium fluorrichterite as the predominant crystal phase, but which also contain at least 10% of cristobalite crystallization. Glass-ceramics included within that disclosure satisfying the above criteria have been developed through the heat treatment of precursor glass articles having the compositions recited below, expressed in terms of weight percent on the oxide basis, of:
______________________________________ SiO.sub.2 65-69 K.sub.2 O 4.2-6 Al.sub.2 O.sub.3 0.75-3 Li.sub.2 O 0.5-2 MgO 13.5-16.5 BaO 0-2 CaO 3-4.8 P.sub.2 O.sub.5 0-2 Na.sub.2 O 1.5-3.3 F 3.3-5 ______________________________________
Whereas glass-ceramics satisfying the seven criteria for substrates to be used in rigid information discs have been prepared and tested from each of the three above-described composition families, glass-ceramics wherein canasite constitutes the predominant crystal phase have been adjudged to be the most preferred for that application.
As is explained in U.S. Pat. No. 4,971,932, the conventional rigid information disc comprises a laminate consisting of at least three layers, viz., a substrate and a layer of magnetic alloy joined to the substrate through an undercoat. Protective and/or lubricating coatings are customarily applied over the layer of magnetic alloy. These layers are collectively termed "media". Cobalt-based alloys have typically comprised the magnetic media and a chromium film sputtered onto the substrate has commonly constituted the underlayer. The substrate most commonly used commercially has comprised a thin disc of aluminum metal. Prior to applying the layers of media, the aluminum substrate must be carefully processed (including electroless NiP plating) to assure flat, non-corroding surfaces with precisely controlled textures.
In addition to the fact that those multi-step processes incur added expense and process control requirements, some inherent limitations of aluminum-based substrates have led media and disc drive manufacturers to look to potential alternative substrate materials, in particular glasses and glass-ceramics. Recognized limitations of aluminum alloys include a tendency toward corrosion if there are any flaws in the protective NiP coating, as well as a minimum disc thickness limitation due to the ease with which aluminum bends because of its ductility and malleability. Alternative materials can offer a number of advantages over aluminum-based substrates, including fewer process steps (no NiP coatings are demanded, for example) and the ability to be mass produced with very flat, smooth, and uniform surfaces.
The canasite-containing glass-ceramic materials referred to above in U.S. Pat. No. 4,386,162 offer a unique set of properties for the substrate application. Their relatively high elastic moduli, coupled with high body strength and toughness, permit the manufacture and use of exceedingly thin (&lt;0.5 mm), yet rugged parts. Furthermore, their microstructures provide an inherent tailorable surface texture. Thus, neither NiP coatings nor separate texturizing processes are required and the substrate is reworkable, i.e., the magnetic coatings may be removed and the disc re-processed, each of those factors involving significant cost savings.
One problem that has been encountered with discs utilizing substrates prepared from any glass-based materials has been the difficulty in achieving optimal magnetic performance. Thus, coercivities of magnetic films processed in the same manner onto glass-based discs have typically been about 10% lower than those measured on discs using NiP-plated aluminum substrates. Laboratory investigation has demonstrated that this difference in magnetic properties is a function of the difference in thermal properties existing between aluminum metal and glass-based materials. That is, glass-based materials do not achieve the same temperature as aluminum-based materials at the film deposition station, the significance of that factor being explained below.
Extensive field experience has demonstrated that the performance of the cobalt-based alloys, i.e., their coercivity, high frequency signal amplitude, bit shift, pulse width, and signal-to-noise ratio thereof, is dependent upon the orientation of the cobalt-based alloy. That orientation can be controlled through the orientation of the chromium underlayer. It has been found that the optimum performance of the cobalt-based alloy takes place when the texture of the chromium film is (100). [S. L. Duan et al., "Study of the Growth Characteristics of Sputtered Cr Thin Films", Journal of Applied Physics, 67, (9), pp. 4913-15, May 1, 1990] The crystallographic orientation of the chromium film, however, is dependent upon the temperature of the substrate during deposition thereof, and, for the film thicknesses conventionally employed in underlayers, the texture becomes predominantly (100) when the temperature of the substrate exceeds 200.degree. C. Consequently, a substrate temperature above 200.degree. C. at the time the chromium coating is applied comprises a prerequisite for achieving satisfactory magnetic performance from cobalt-based alloys irrespective of the substrate composition.
In many of the conventional commercial production processes for preparing rigid information discs, heating of the substrate is accomplished via the absorption of radiation from a tungsten-halogen lamp in a vacuum system. Nickel phosphorus plated aluminum (NiP/Al) substrates can be heated in this manner to temperatures in excess of 220.degree. C. in less than about 15 seconds. Depending upon its microstructure, the amount of radiation absorbed from such a lamp by canasite can be substantially less than that absorbed by NiP/Al. Accordingly, in any given apparatus, canasite can require longer heating times. In addition, cooling of the substrate which occurs during film processing/deposition as the substrate is transferred from the heating station to the deposition station in commercial sputtering units is greater for substrates prepared from glasses or glass-ceramic than from NiP/Al-based substrates. And, inasmuch as media suppliers are quite reluctant to alter their processes, in particular to processes which require longer periods of time and, consequently, add cost, acceptance in the marketplace of glass-based materials in the fabrication of substrates for rigid information discs has been limited, even though they exhibit several properties superior to those found in NiP/Al.
Therefore, the principal objective of the present invention was to develop means for making glass-based materials capable of attaining temperatures similar to those attained by NiP/Al during deposition of the film.
A specific objective of the present invention was to develop means for making canasite-containing glass-ceramic substrates for rigid information discs having the capability of attaining temperatures similar to that of NiP/Al substrates during deposition of the film and to retain that temperature in a manner similar to NiP/Al substrates.